libcruft-util/colour.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2010-2021 Danny Robson <danny@nerdcruft.net>
 */

#include "colour.hpp"

#include "ascii.hpp"
#include "parse/value.hpp"
#include "debug/assert.hpp"

using cruft::colour;
using cruft::srgba;


///////////////////////////////////////////////////////////////////////////////
static cruft::srgba4f
parse_hex (cruft::view<const char*> &str)
{
    if (str.size () != strlen ("#012345"))
        throw std::invalid_argument ("expected length of 7");

    if (str[0] != '#')
        throw std::invalid_argument ("expected leading '#'");

    if (!cruft::ascii::is_hex (str[1]) ||
        !cruft::ascii::is_hex (str[2]) ||
        !cruft::ascii::is_hex (str[3]) ||
        !cruft::ascii::is_hex (str[4]) ||
        !cruft::ascii::is_hex (str[5]) ||
        !cruft::ascii::is_hex (str[6]))
    {
        throw std::invalid_argument ("expected hex digits");
    }

    uint8_t r = cruft::ascii::from_hex (str[1]) << 4u | cruft::ascii::from_hex (str[2]);
    uint8_t g = cruft::ascii::from_hex (str[3]) << 4u | cruft::ascii::from_hex (str[4]);
    uint8_t b = cruft::ascii::from_hex (str[5]) << 4u | cruft::ascii::from_hex (str[6]);

    str = str.consume (7u);
    return cruft::srgba<4,uint8_t> { r, g, b, 255 }.template cast<float> ();
}



///////////////////////////////////////////////////////////////////////////////
template <>
cruft::srgba4f
cruft::parse::value<cruft::srgba4f> (cruft::view<const char*> &str)
{
    return parse_hex (str);
}


///////////////////////////////////////////////////////////////////////////////
/// Invert the sRGB gamma function
static constexpr float
linearise (float const u)
{
    if (u <= 0.04045f)
        return u / 12.92f;
    else
        return std::pow ((u + 0.055f) / 1.055f, 2.4f);
}


///----------------------------------------------------------------------------
/// Apply the sRGB gamma function
static constexpr float
exponentiate (float const u)
{
    if (u <= 0.0031308f)
        return 12.92f * u;
    else
        return 1.055f * std::pow (u, 1/2.4f) - 0.055f;
}


///----------------------------------------------------------------------------
/// Convert an sRGB colour to XYZ.
///
/// Gamma will be removed during the conversion. Do not use with linear RGB.
static cruft::vector3f
sRGB_to_XYZ (cruft::vector3f sRGB)
{
    auto const [r_, g_, b_] = sRGB;

    float const r = linearise (r_);
    float const g = linearise (g_);
    float const b = linearise (b_);

    // D65 whitepoint, 2° observer.
    //
    // We use higher precision values than the specification dictates, from
    // Bruce Lindbloom's reference, so as to minimise numerical errors when
    // chaining conversions.
    float const X = r * 0.4124564f + g * 0.3575761f  + b * 0.1804375f;
    float const Y = r * 0.2126729f + g * 0.7151522f  + b * 0.0721750f;
    float const Z = r * 0.0193339f + g * 0.1191920f  + b * 0.9503041f;

    return {X, Y, Z};
}


///----------------------------------------------------------------------------
/// Convert an XYZ colour to sRGB.
///
/// Gamma will be applied during the conversion.
///
/// Out of range colours will be returned as is. It is the caller's
/// responsibility to process these as desired.
static cruft::vector3f
XYZ_to_sRGB (cruft::vector3f XYZ)
{
    auto const [X, Y, Z] = XYZ;

    float const r = +3.2404542f * X -1.5371385f * Y -0.4985314f * Z;
    float const g = -0.9692660f * X +1.8760108f * Y +0.0415560f * Z;
    float const b = +0.0556434f * X -0.2040259f * Y +1.0572252f * Z;

    return {
        exponentiate (r),
        exponentiate (g),
        exponentiate (b),
    };
}


///----------------------------------------------------------------------------
/// Convert an XYZ colour to xyY
static
cruft::vector3f
XYZ_to_xyY (cruft::vector3f XYZ)
{
    auto const [X, Y, Z] = XYZ;

    if (cruft::abs (X + Y + Z) < 1e-5f) {
        // Use the D65 xy if we're at black.
        return {
            0.31271f,
            0.32902f,
            Y
        };
    } else {
        return {
            X / (X + Y + Z),
            Y / (X + Y + Z),
            Y,
        };
    }
}


///----------------------------------------------------------------------------
/// Convert an xyY colour to XYZ
static cruft::vector3f
xyY_to_XYZ (cruft::vector3f xyY)
{
    auto const [x, y, Y] = xyY;
    if (cruft::abs (y) < 1e-5f) {
        return {0, 0, 0};
    } else {
        return {
            x * Y / y,
            Y,
            (1 - x - y) * Y / y,
        };
    }
}


//-----------------------------------------------------------------------------
cruft::srgba3f
cruft::scale_luminance (cruft::srgba3f sRGB, float f)
{
    // We can't scale sRGB directly. Given we probably have to convert to other
    // spaces via XYZ anyway we may as well scale the luminance value in xyY.
    //
    // The conversion chain is thus:
    //    * sRBB -> XYZ -> xyY -> scaling -> XYZ -> sRGB

    auto const XYZ0= sRGB_to_XYZ (sRGB.as<cruft::vector3f> ());
    auto const xyY0= XYZ_to_xyY (XYZ0);

    auto const xyY1 = xyY0 * cruft::vector3f{1,1,f};
    auto const XYZ1 = xyY_to_XYZ (xyY1);
    return XYZ_to_sRGB (XYZ1).as<cruft::srgba3f> ();
}


//-----------------------------------------------------------------------------
cruft::srgba4f
cruft::scale_luminance (cruft::srgba4f sRGBA, float f)
{
    auto const sRGB0 = sRGBA.indices<0,1,2> ();
    auto const sRGB1 = scale_luminance (sRGB0, f);

    return {
        sRGB1.r,
        sRGB1.g,
        sRGB1.b,
        sRGBA.a,
    };
}